SYN_STREAM,SYN_REPLY,HEADERS => HEADERS+PRIORITY,HEADERS

This reduces the number of frames as per HTTP/2.0 draft changes. It also
has the following effects:
* Eliminates some RST_STREAM error codes
* Eliminates the unidirectional stream stuff like HTTP/2.0
* Fixes up some of the push text to say PUSH_PROMISE instead of
  SYN_STREAM
* Removes the unused FLAG_HEADER_END
Merge pull request #12 from willchan/HeadersFrames

SYN_STREAM,SYN_REPLY,HEADERS => HEADERS+PRIORITY,HEADERS